Method for grading the screen of a color tube

ABSTRACT

In the formation of a screen for a color cathode ray tube, a method for exposing a photosensitive coating on a cathode ray tube faceplate comprises illuminating the coating with radiation actinic to said coating emanating from an effective point source through a shadow mask disposed adjacent to said coating having a predetermined pattern of apertures formed therein so as to impress on the coating a corresponding pattern of latent aperture images. The method also includes altering the energy distribution pattern impressed on the coating through the mask by imposing thereon a two-dimensional, time-varying shading function.

United States Patent [1 1 Park et a]. Oct. 15, 1974 [54] METHOD FORGRADING THE SCREEN 0F 1259,0324 v/lwm Burdick cl alt 4. 05/! R A COLORTUBE 3.6745388 7/1972 Dodd cl aI....I.. 96/44 3,698,903 lO/l972 D )dd t1!. 96/44 [75] Inventors: Yong S. Park, Hanover Park; L I

: y mend Pekosh Nlles both of Primary Examiner-David Klein AssistantExaminer-Edward C. Kimlin [73] Assignee: Zenith Radio Corporation,Chicago Attorney, Agent, or Firm-John H. Coult Ill.

[22] Filed: June 4, I973 [57] ABSTRACT [21] Appl. No; 366,397 In theformation of a screen for a color cathode ray Related U S A cation Datatube, a method for exposing a photosensitive coating 6 pp on a cathoderay tube faceplate comprises illuminating 2] g 1972' the coating withradiation actinic to said coating emanating from an effective pointsource through a shadow mask disposed adjacent to said coating having[52] US. Cl. 96/36.l, 96/27 E, 3555/51/51, a predetermined pattern ofapertures formed therein so as to impress on the coating a correspondingpatg tern of latent aperture images. The method also inle 0 can 95/6355/71 eludes altering the energy distribution pattern impressed on thecoatin throu h the mask by imposin h d l h d' f g t ereon a twoimensionatime-varying s a mg unc- [5 6] References Cited tiom UNITED STATESPATENTS 3,146,368 8/1964 Fiore 96/36.l 12 Clam, 3 Dmwmg Flgures METHODFOR GRADING THE SCREEN OF A COLOR TUBE RELATED APPLICATIONS Thisapplication is a division of application Ser. No. 248,751, filed Apr.28, 1972, now U.S. Pat. No. 3,762,289, and assigned to the assignee ofthis application. This application is related to an application entitledA RECTANGULAR GRADE BLACK SUR- ROUND SCREEN in the name of LeonardDietch, filed Apr. 4, I972, Ser. No. 240,93l, now U.S. Pat. No.3,790,839, and assigned to the assignee of the present invention.

BACKGROUND OF THE INVENTION The present invention concerns the gradepattern of viewable image elements or phosphor deposits on the faceplateof a color television picture tube. The image elements in a conventionaltri-gun shadow mask color cathode ray tube comprise deposits of lightemitting phosphors having different color responses. The term viewable"indicates the maximum area of lightemitting surface, which in aconventional positive guardband tube is the area of the electron beamimpinging thereon.

During screening of the color tube faceplate, successive exposuresthrough the shadow mask, are made in a lighthouse. The exposures aretaken with the faceplate and exposure source occupying relativelydifferent positions, corresponding to the different electron gunpositions in the finished tube, with the faceplate coated with aphotosensitive slurry bearing a phosphor of an appropriate primarycolor. The size of the ultimate phosphor deposits on the faceplate is inpart determined by the length and intensity of exposure, and undernormal conditions, the phosphor deposits are larger than the projectedareas of the corresponding apertures in the shadow mask.

Recently, color television picture tubes having elemental phosphordeposits separated from one another over the faceplate, with the spacesbetween phosphor deposits filled with a light absorptive material, havefound widespread commercial acceptance. This type of color televisionpicture tube, commonly referred to as a black surround tube, is fullydescribed in U.S. Pat. No. 3,146,368 issued Aug. 25, 1964, in the nameof .loseph P. Fiore et al, and assigned to the assignee of the presentinvention. In one method of manufacturing a black surround tube, amatrix of opaque, non-reflective material is initially deposited on theinner surface of the faceplate. The matrix defines a plurality ofphosphor receiving holes arranged in triads, which holes determined theviewable area of the latter-applied phosphor deposits, since the blacksurround is opaque. Thus, the maximum light emitting area is restrictedby the black surround hole area even though the cross section area ofthe electron beam impinging on the phosphor deposit may be greater thanthat of the hole. In such tubes, the purity tolerance or guard band maybe reversed with numerous advantages in brightness and contrastcharacteristics as fully set out in the abovementioned Fiore et al,patent. Thus, a negative tolerance of guard band means that the electronbeam area is larger than the effective phosphor dot area. The negativetolerance may be photographically achieved as set out in the Fiore et alpatent or may be attained by enlarging the shadow mask apertures afterscreening has been performed.

As indicated in the aforementioned co-pending Dietch application, ablack surround with holes having areas graded in accordance with arectangular pattern affords desirable benefits in terms of brightnessand guard band. The areas of the holes in the black surround may becontrolled by appropriately shading the faceplate with itsphotosensitized coating during exposure since the sizes of the imagesprojected through the shadow mask apertures are a function of exposureintensity and duration. Particularly, a shader" plate comprising a glassplate or other transparent substrate having a deposited metal of microthickness for attenuating the exposure rays may be employed in thelighthouse between the exposure source and shadow mask. The intensitydistribution of the exposure energy can thus be controlled to achievethe desired hole area grading pattern.

Accordingly, it is an object of the invention to provide an improvedmethod of screening a color cathode ray tube of the black surround type.

It is a specific object of the invention to provide an improved methodof screening color cathode ray tube which enhances purity.

Another object of this invention is to provide a novel method ofproducing a color tube screen having viewable image elements diminishingin area from the center to the edge and characterized in that imageelements of similar area are disposed along a substantially rectangularlocus centered with respect to the faceplate.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention is defined withparticularity in the appended claims. The invention, together withfurther objects and advantages thereof, may best be understood byreference to the following description taken in connection with theaccompanying drawing in the several figures of which like referencenumerals identify like elements, and in which:

FIG. 1 is a cross sectional view of a lighthouse incorporating shadingmechanism executing the method of the invention;

FIG. 2 is a view of the shading mechanism taken along the lines 22 ofFIG. 1; and

FIG. 3 is an exploded perspective view of a portion of the shadingmechanism.

DESCRIPTION OF THE PREFERRED METHOD The drawings and ensuing textdescribe apparatus which performs a preferred execution of the inventivemethod. Referring to FIG. 1, a lighthouse 10 including a housing 11, alight assembly 12 and an exposure surface 16, is shown. The faceplate orpanel 20 of a conventional color picture tube is shown mounted inposition'on exposure surface 16. A conventional aperture type shadowmask2l is mounted closely adjacent the inner surface of panel 20. Theinner surface of faceplate 20 is conventionally coated with aphotosensitive material such as polyvinyl alcohol (pva) sensitized withammonium dichromate, whose solubility in water is changed in response toultraviolet light. Light assembly 12 includes an ultraviolet lightsource 13, a collimator l4 and a lens 15. Collimator l4 emits theultraviolet rays at its tip and is generally designed to simulate apoint source. Lens 15 is indicated in block form and may comprise any ofa well known number of configurations for displacing the light raysfalling on faceplate 20 to compensate for the differences intrajectories be tween lightrays and electron beams in a color tube.Lighthouse may be conventional in all respects with the exception ofshadingmechanism 50 located between light source 13 and the exposuresurface 16.

Conventional photosensitive coatings such as pva sensitized withammonium dichromate respond to ultraviolet light in a manner determinedby both the exposure duration and intensity. Generally speaking, alonger exposure or a higher intensity exposure results in a larger areaof insolubilized pva. Consequently, if the shadow mask apertures arecircular, the latent images of those apertures formed in the coatingwill be circular, but of a size determined by the intensity and durationof exposure.

While the tip of the collimator simulates a point source of light andradiates substantially equally in all directions throughout thedeflection angle of the color tube, a peaking of intensity in thecentral area of the faceplate occurs because the tip is not located atthe center of the radius of curvature of the faceplate. Thus, theintensity distributionof the light energy falling on the inner surfaceof faceplate decreases from center to edge. In a rectangular panel, theextremities of the diagonals are farthest from the light source and,therefore, the intensity is lowest. This type of energy distributionwill not result in the desirable rectangular grade pattern for theexposed areas of the coating.

The shading mechanism for obtaining the desired light distribution willbest be understood by referring to FIGS. 1, 2 and 3 together. As shownin FIG. 1, a support plate 17 is mounted to the lower portion oflighthouse 11 and supports a bearing 18. A cam plate support 29 ismounted for rotational movement on bearing 18. A movable cam plate 30 iseither attached to cam 5 plate support 29 or maybe formed integrallytherewith.

A stationary drive plate 40 is affixed to housing 11 in overlyingrelationship with cam plate 30. A pair of vertical shading plates 61 aremounted in drive plate 40 for rectilinear movement toward and away fromeach other along a path parallel to the major axis of faceplate 20. Fourrack supports 56 are fastened about the circumference of drive plate 40and partially overlie vertical shading plates 61. The rack supportscarry a pair of horizontal shading plates 51 which similarly are mountedfor rectilinear motion toward and away from each other, but along a pathparallel to the minor axis of faceplate 20. Each rack support includes arack 57 with a mating pinion 55, mounted on a shaft 54, which isjournalled in a pair of bearing supports 53 affixed to the horizontalshading plates.

A stationary guide plate 40 includes a plurality of guide slots, two foreach vertical shading plate and one for each horizontal shading plate.The vertical guide slots 42 and the horizontal guide slots 41 areadapted to receive vertical guide pins 63 and the horizontal guide pins58, respectively, on the bottom of each shading plate. Horizontal guidepin 58 has an extension terminating in a horizontal drive pin 59engaging a horizontal cam 32 in cam plate 30. While each verticalshading plate 61 has two pairs of guide pins 63, only one of the guidepins has an extension terminating in a vertical drive pin 64 engaging avertical cam 33 in cam plate 30.

Movable cam plate 30 includes a partial gear ring 31 attached to, orformed as part of its circumference. A reversible motor 25, mounted fromthe lighthouse by a bracket 19, powers a drive gear 26 in engagementwith gear ring 31. A cycling pin 34 is mounted to the circumference ofcam plate 30 for alternately engaging and operating a pair of reversingswitches 35 and 36 which are shown affixed to stationary guide plate 40.The reversing switches change the direction of rotation of motor 25 in acyclical manner, and drive gear 25 rotates cam plate 30 back and forthbetween the positions determined by the switches.

Horizontal earns 32 comprise arcuate slots in cam plate 30, engaged byhorizontal drive pins 59 attached to the underside of shading plate 51.The vertical cams 33 approximate L-shaped slots in cam plate 30 engagedby vertical drive pins 34 attached to the underside of shading plates61. As cam plate 30 rotates clockwise, horizontal drive pins 59 areforced inward by the action of horizontal cams 32. The line of travel ofhorizontal shading plates 51 is controlled by the action of guide pin 58in guide slot 41 and the provision of the rack and pinion arrangementwhich precludes all but rectilinear movement. Vertical drive pins 64remain substantially stationary during the first portion of clockwisemovement of cam plate 30 while the pins are in the arcuate shaped dwellportions of the vertical cams indicated by 33d. Responsive to continuedrotation of cam plate 30, drive pins 63 are driven rapidly inward by theeffective portions of the vertical cams indicated by 33c. Thus. as camplate 30 rotates, horizontal shading plates 51 are brought towards eachother at a constant rate, whereas the vertical plates are initiallystationary and subsequently moved toward each other at a faster rate.When moved, the shading plates shadow portions of faceplate 20 from thelight source.

When cam plate cycling pin 34 operates switch 36, motor reversal occursand the cam plate is driven clockwise. Actions now reverse with thevertical shading plates being driven away from each other rapidlywhereas the horizontal plates are driven at a slower rate.

The horizontal and vertical shading plates cooperate to define a windowor opening whose shape and area vary with time. This window may beregarded and described as representing a two-dimensional, timevaryingshading function. It is apparent that the vertically moving horizontalshading plates and also the horizontally moving vertical shading plateseach accomplish an area-wise shading of the photosensitive coating onthe faceplate, i.e., a shading in two dimensions. Each of the shadingplate pairs alters the energy distribution pattern impressed on thephotosensitive coating, the horizontal shading plates acting along avertical axis and the vertical shading plates acting along a horizontalaxis.

By shaping the frontal edge of each plate, for example parabolically, asshown, additional and compensating light energy is impressed along thefaceplate diago' nals. This is seen more clearly in FIG. 2 where thefour plates cooperate to form a time-varying window of pincushion shape.The window corners cause the portions of the faceplate along thediagonals to be exposed for a longer period of time than the portionsalong horizontal and vertical axes.

In a preferred embodiment, the plates are shaped to resemble parabolasand their movements timed to produce a light distribution pattern overthe faceplate for yielding a locus of rectangles of similar area images,with the image areas diminishing from center to edge, centered about thefaceplate. The continual movement of the plates produces gradual lightdistribution changes from point-to-point across the faceplate and thusresults in a uniform transition in image area size.

The provision of four vertical guide pins 63 cooperating with a pair ofparallel guide slots 32 precludes the vertical shading plates fromtraveling in other than a rectilinear path. It will be appreciated thatthe guidance arrangement for the vertical plates and for the horizontalplates may be used interchangeably, the criterion being that freedomfrom side movement be maintained.

It will be recognized that the size and shape of the plates may bevaried along with timing of plate operations to produce a variety ofshading patterns. This is of particular importance in a production areawhere adjustments may be made to compensate for the innumerablevariables encountered in screening operations. This flexibility is notobtainable with fixed shader plates or lens coatings and is an extremelydesirable attribute of the invention.

While particular methods of the invention have been shown and described,it will be obvious to those skilled in the art that changes andmodifications may be made without departing from the invention in itsbroader aspects, and, therefore, the aim in the appended claims is tocover all such changes and modifications as fall within the true spiritand scope of the invention.

, What is claimed is:

1. In the formation of a screen for a color cathode ray tube, a methodfor exposing a photosensitive coating on a cathode ray tube faceplatecomprising illuminating the coating with radiation actinic to saidcoating emanating from an effective point source through a shadow maskdisposed adjacent to said coating having a predetermined pattern ofapertures formed therein so as to impress on the coating a correspondingpattern of latent aperture images, said method including altering theenergy distribution pattern impressed on the coating through the mask byshading said coating as a function of (i) time, (ii) the radial distanceof latent images tst tx a sa at se tt n a (ii) tha l flt a easlwt latentimages on the faceplate, such that there is defined on the faceplatecoating a plurality of noncircular loci wherein each said non-circularlocus consists of latent images which receive an equal time of exposure,wherein said shading comprises casting two shadow patterns on said maskwhich move with time across said mask in substantially orthogonaldirections, each of said shadow patterns comprising first and secondportions moving with time along the same axis in opposite directions.

2. The method defined by claim 1 wherein said loci are substantiallyrectangular.

3. The method defined by claim 2 wherein said loci are substantiallyconcentric and centered about said faceplate, and the exposure times ofsuccessive loci di minish from faceplate center to faceplate edge.

4. The method defined by claim 1 wherein the confronting edges of thefirst and second portions of at least one shadow pattern are parabolic.

5. The method defined by claim I wherein the two shadow patterns createa border around a non-shaded area of said mask and said non-shaded areavaries in size and shape with time.

6. The method defined by claim 5 wherein the nonshaded area exhibits apincushion shape.

7. The method defined by claim 1 wherein said shading varies cyclicallywith time.

8. The method of manufacturing a rectangular screen assembly for a blacksurround color tube comprising the steps of:

a. applying a coating of photosensitive material to the faceplate of thetube;

b. mounting a shadow mask with a pattern of aper tures in spacedrelationship to the faceplate;

c. providing a lighthouse having an exposure surface. a source of energyactinic to said photosensitive coating and an automatic shadingmechanism between the energy source and the exposure surface, saidshading mechanism casting two shadow patterns on said mask which movewith time across said mask in substantially orthogonal directions, eachof said shadow patterns comprising first and second portions moving withtime along the same axis in opposite directions;

(1. mounting the faceplate and shadow mask on the exposure surface;

e. exposing the photosensitive coating from the source of energy throughthe shadow mask, while varying the time of exposure with said automaticshading mechanism such that time of exposure of coating elementsassociated with said mask apertures depends upon the radial distance ofcoating elements from faceplate center and upon azimuthal angle ofcoating elements on the faceplate, such that there is defined on thefaceplate coating a plurality of substantially rectangular, faceplatecentered, and concentric loci wherein each said locus consists ofcoating elements which receive an equal time of exposure;

f. developing the latent image of said pattern of apertures formed insaid coating; and

g. applying a black pigment to the non-exposed portions of saidfaceplate.

9. The method set forth in claim 8 wherein said exposure step comprisesrepetitive exposures of said coating through said shadow mask with saidenergy source located in different positions corresponding to the threeelectron guns in a color tube.

10. The method set forth in claim 9 wherein said automatic shadermechanism comprises a set of plates, movable during exposure, forproviding said variable time of exposure.

11. The method set forth in claim 10 wherein said set of plates includea pair of horizontal plates movable substantially along a path parallelto the minor axis of said faceplate and a pair of vertical platesmovable substantially along a path parallel to the major axis of saidfaceplate.-

12. The method set forth in claim ll wherein control means are providedfor moving said horizontal and vertical plates at different rates.

1. IN THE FORMATION OF A SCREEN FOR A COLOR CATHODE RAY TUBE, A METHODFOR EXPOSING A PHOTOSENSITIVE COATING ON A CATHODE RAY TUBE FACEPLATECOMPRISING ILLUMINATING THE COATING WITH RADIATION ACTINIC TO SAIDCOATING EMANATING FROM AN EFFECTIVE POINT SOURCE THROUGH A SHADOW MASKDISPOSED ADJACENT TO SAID COATING HAVING A PREDETERMINED PATTERN OFAPERTURES FORMED THEREIN SO AS TO IMPRESS ON THE COATING A CORRESPONDINGPATTERN OF LATENT APERTURE IMAGES, SAID METHOD INCLUDING ALTERING THEENERGY DISTRIBUTION PATTERN IMPRESED ON THE COATING THROUGH THE MASK BYSHADING SAID COATING AS A FUNCTION OF (I) TIME, (II) THE RADIAL DISTANCEOF LATENT IMAGES FROM FACEPLATE CENTER, AND (II) THE AZIMUTHAL ANGLE OFLATENT IMAGES ON THE FACEPLATE, SUCH THAT THREE IS DEFINED ON THEFACEPLATE COATING A PLURALITY OF NON-CIRCULAR LOCI WHEREIN EACH SAIDNON-CIRCULAR LOCUS CONSISTS OF LATENT IMAGES WHICH RECEIVE AN EQUAL TIMEOF EXPOSURE, WHEREIN SAID SHADING COMPRISES CASTING TWO SHADOW PATTERNSON SAID MASK WHICH MOVE WITH TIME ACROSS SAID MASK IN SUBSTANTIALLYORTHOGONAL DIRECTIONS, EACH OF SAID SHADOW PATTERNS COMPRISING FIRST ANDSECOND PORTIONS MOVING WITH TIME ALONG THE SAME AXIS IN OPPOSITEDIRECTIONS.
 2. The method defined by claim 1 wherein said loci aresubstantially rectangular.
 3. The method defined by claim 2 wherein saidloci are substantially concentric and centered about said faceplate, andthe exposure times of successive loci diminish from faceplate center tofaceplate edge.
 4. The method defined by claim 1 wherein the confrontingedges of the first and second portions of at least one shadow patternare parabolic.
 5. The method defined by claim 1 wherein the two shadowpatterns create a border around a non-shaded area of said mask and saidnon-shaded area varies in size and shape with time.
 6. The methoddefined by claim 5 wherein the non-shaded area exhibits a pincushionshape.
 7. The method defined by claim 1 wherein said shading variescyclically with time.
 8. The method of manufacturing a rectangularscreen assembly for a black surround color tube comprising the steps of:a. applying a coating of photosensitive material to the faceplate of thetube; b. mounting a shadow mask with a pattern of apertures in spacedrelationship to the faceplate; c. providing a lighthouse having anexposure surface, a source of energy actinic to said photosensitivecoating and an automatic shading mechanism between the energy source andthe exposure surface, said shading mechanism casting two shadow patternson said mask which move with time across said mask in substantiallyorthogonal directions, each of said shadow patterns comprising first andsecond portions moving with time along the same axis in oppositedirections; d. mounting the faceplate and shadow mask on the exposuresurface; e. exposing the photosensitive coating from the source ofenergy through the shadow mask, while varying the time of exposure withsaid automatic shading mechanism such that time of exposure of coatingelements associated with said mask apertures depends upon the radialdistance of coating elements from faceplate center and upon azimuthalangle of coating elements on the faceplate, such that there is definedon the faceplate coating a plurality of substantially rectangular,faceplate centered, and concentric loci wherein each said locus consistsof coating elements which receive an equal time of exposure; f.developing the latent image of said pattern of apertures formed in saidcoating; and g. applying a black pigment to the non-exposed portions ofsaid faceplate.
 9. The method set forth in claim 8 wherein said exposuresTep comprises repetitive exposures of said coating through said shadowmask with said energy source located in different positionscorresponding to the three electron guns in a color tube.
 10. The methodset forth in claim 9 wherein said automatic shader mechanism comprises aset of plates, movable during exposure, for providing said variable timeof exposure.
 11. The method set forth in claim 10 wherein said set ofplates include a pair of horizontal plates movable substantially along apath parallel to the minor axis of said faceplate and a pair of verticalplates movable substantially along a path parallel to the major axis ofsaid faceplate.
 12. The method set forth in claim 11 wherein controlmeans are provided for moving said horizontal and vertical plates atdifferent rates.